Many of the passive vehicle occupant restraint systems that have been proposed to improve safety and meet regulations which are likely to be adopted rather soon have one or more movable guide rings to transfer a portion or portions of a seat or shoulder belt, or both, between an occupant-releasing configuration and an occupant-restraining configuration. For example, the system may have a shoulder belt leading from a retractor mounted on the roof near the upper rear corner of the vehicle door to and through a movable guide ring which moves along a guide rail installed in the roof above the door and then across to an anchor. Similarly, a lap belt may lead from a retractor affixed to the lower rear corner of the door to and through a guide ring that moves along a rail extending diagonally up to a position near the front end of the window sill of the door and then across to an anchor on the inboard side of the seat. Each time the belts are transferred between the releasing and restraining configurations, they slide through the guide rings, and even though the rings have smooth surfaces, it is inevitable that the constant scuffing that occurs wears the belts. Moreover, it is necessary to provide a belt sufficiently long to allow a section to be unwound from the retractor so that the belt can attain the releasing configuration; this means that the retractor must be of a larger size than required for the restraint configuration.
In attempt to overcome the foregoing and other problems, there have also been proposals for passive systems in which the end of a belt is attached to a movable anchor. The movable anchor systems proposed thus far, however, have still presented problems, such as overly-complicated mechanisms for driving and locking the anchor, high manufacturing costs, and difficulties involved in establishing and maintaining the position of the anchor in the passenger-restraining configuration. The location of the moving anchor in the restraint position is particularly difficult in the case of hardtop passenger cars, which do not have a center post.